Video and still picture data is generated in a variety of aspect ratios. Conventional NTSC televison uses a 4:3 horizontal to vertical frame ratio. High Definition Television (HDTV) uses a 16:9 ratio. Other ratios are also commonly used by motion pictures. Still pictures have many formats depending on the equipment, cameras etc. used to generate, including square, portrait, landscape, and many others. The MPEG Video Standard ISO/IEC 13818-2 of November 1994, for example lists among others, a 711:483 pixel display size and a 702:575 display size.
When a frame having one aspect ratio is displayed on a display device having a different aspect ratio, some type of accommodation must be made.
One type of accommodation known in the art involves stretching e.g. magnifying or demagnifying either the horizontal or vertical dimension to make the generated frame completely fill the display device area. For example a 4:3 source may be stretched or scaled up horizontally to completely fill a 16×9 HDTV display. It is well known that such stretching causes the image to be distorted so that for example circles will appear as ellipses. Faces take on an unnatural appearance.
The aspect ratio differences may also be accommodated by keeping the source aspect ratio fixed and utilizing only a part of the display device area. Typically one dimension is fully utilized and the other dimension will have blank areas. If the source is centered on the display device, then there will be blank areas at the top and bottom of the screen, known as letterbox, or else there will be blank areas at the left and right sides. Wang in U.S. Pat. No. 5,581,304 describes a method of detecting a 4×3 letterbox frame having blank top and bottom areas. If this frame is to be displayed on a 16×9 display device, the blank areas may be detected and removed to restore the aspect ratio to 16×9 for proper display on the 16×9 device. Viewers having a 4×3 display device will see the full letterbox display including blank areas.
Another accommodation method known in the art involves display of a 16×9 source on a 4×3 display device. The 16×9 source is cropped to remove some of the source area. Typically the center area is kept, however any portion may be selected depending on which part is more important. This method is referred to as pan-scan because the most important part may change over time producing a horizontal panning effect as the image is shifted horizontally on the 4×3 display device.
Washino in U.S. Pat. No. 5,999,220 describes a system and method for converting video data between the various formats just described. Haruki in Japanese patent JP04345388A abstract describes aspect ratio conversion in a wide screen televison receiver.
As noted some of the methods leave blank areas on the display device when displaying certain aspect ratio source data. It is believed that a better utilization of these blank areas would be a desirable feature contributing to advancement in the art.